1. Field of the Invention
The present invention generally relates to a mobile communication system and a mobile communication method. More specifically, the present invention is directed to a mobile communication system and a mobile communication method, capable of selecting a wireless base station functioning as a hand-over destination based on a neighbor list when a hand-over operation occurs.
2. Description of the Related Art
Mobile communication systems constructed of a plurality of base stations capable of covering a service area are widely used in the technical field. A mobile station and a base station communicate with each other via a communication link.
While a communication link is established between a certain mobile station and a certain base station, the communication link is required to be interrupted, or cut off, depending upon a condition of the communication link. In the case that a communication link between the mobile station and another base station may be established under better condition, another communication link must be established between the mobile station and the other base station.
The methods for interrupting the communication link between the mobile station and the base station, or for establishing the communication link between another base station and the mobile station are known from, for example, Japanese Laid-open Patent Applications (JP-A Heisei 5-316023, JP-A Heisei 7-307972, JP-A Heisei 9-284828, and JP-A Heisei 10-248090), and Japanese Patent Office Gazette (2710180).
When a communication link between a mobile station and a base station is established, the base station may be determined with reference to a neighbor list. A neighbor list implies such a list indicative of base stations which may constitute base station subjects capable of establishing communication links with a mobile station. Base station codes specific to the respective base stations are given to these base stations. The neighbor list is constituted by base station codes of base stations which constitute base station subjects for establishing communication links.
In general, a neighbor list is notified from a base station to a mobile station. The mobile station receives electromagnetic waves transmitted from base stations having base station codes which are listed in this neighbor list. The mobile station measures electric field intensities of these electromagnetic waves. Based upon measurement results of the electric field intensities, such a base station which will establish a communication link with this mobile station may be determined.
Generally speaking, neighbor lists must be properly produced. In such a case that a neighbor list does not contain such a base station code of an optimum base station when a communication link is established, a mobile station cannot switch a connection to such a base station, while this base station may constitute an optimum base station functioning as a base station when a communication link is established. As a result, this mobile station is forcibly communicated with a not-proper base station. Thus, a communication quality during communication would be lowered. In the worst case, the communication would be interrupted. Also, interference given to other mobile stations may be increased. Therefore, communication qualities of other mobile stations may be lowered.
On the other hand, in such a case that a large number of base station codes of such base stations which are not suitable to establish communication links are contained in a neighbor list, lengthy measuring time of electric field intensities is required. Thus, a time period required to determine a proper base station while a communication link is established would be prolonged. Also, in this case, communication qualities while communications are performed would be lowered. In the worst case, the communication may be cut off.
As a consequence, proper base stations are selected and indicated in such a neighbor list.
Therefore, the present invention has an object to provide a mobile communication system capable of selecting a base station to be connected to a mobile station based upon a properly formed neighbor list.
In order to achieve an aspect of the present invention, a mobile communication system includes a plurality of base stations includes a first and second connecting base stations and a mobile terminal connected to the first connecting base station. Relevant base stations are selected from among the plurality of base stations based on hand-over counts, each of which indicates the number of times of hand-over operations of the mobile terminal from the first connecting base station to one of the plurality of base stations. A second connecting base station is selected from among the relevant base stations based on electric field intensities from the relevant base stations, and the mobile terminal establishes a connecting link with the second connecting base station.
A mobile communication may further include a controlling station. The controlling station sends a neighbor list indicative of the relevant base stations to the mobile terminal. The mobile terminal measures the electric field intensities in response to the neighbor list and sends a electric-field-indicative signal indicative of the electric field intensities to the controlling station. The controlling station selects the second connecting base station in response to the electric-field-indicative signal.
The controlling station may include a hand-over count indicating table indicative of the hand-over counts and produce the neighbor list referring to the hand-over count indicating table.
The hand-over count indicating table may indicate that each of the plurality of base stations is classified into a plurality of groups based on the hand-over counts, and the plurality of groups consists of a dominant group and remaining ones of the plurality of groups. The number of times of communication of the mobile terminal with base stations classified into the dominant group after communicating with the first connecting base station is larger than that with base stations classified into the remaining ones of the plurality of groups. The neighbor list indicates as the relevant base stations all of base stations classified into the first dominant group and include some but not all base stations classified into the remaining ones.
The controlling station may further include a neighbor list generating table including a plurality of lists. The base stations classified into the dominant group are indicated in each of the plurality of lists, and the base stations classified into the remaining ones of the plurality of groups are indicated in one or more but not all of the plurality of lists, and the neighbor list is produced by being selected from among the plurality of lists.
In order to achieve another aspect of the present invention, a controlling station used in a mobile communication system includes a controlling unit. The controlling unit selects relevant base stations from among a plurality of base stations based on hand-over counts, each of which indicates the number of times of communication of a mobile terminal with one of the plurality of base stations after communicating with another base station as a first connecting base station. The controlling unit sends a neighbor list indicative of the relevant base stations to the mobile terminal. And the controlling unit selects a second connecting base station with which the mobile terminal establishes a connecting link from among the relevant base stations based on electric field intensities from the relevant base stations measured by the mobile terminal.
In order to achieve still another aspect of the present invention, a method of operating a mobile communication system includes selecting relevant base stations from among a plurality of base stations based on hand-over counts, each of which indicates the number of times of hand-over operations of a mobile terminal from a first connecting base station to one of the plurality of base stations, selecting a second connecting base station from among the relevant base stations based on electric field intensities from the relevant base stations and establishing a connecting link between the mobile station and the second connecting base station.
In order to achieve yet still another aspect of the present invention, a computer-readable recording medium stores a program for a method which includes selecting relevant base stations from among a plurality of base stations based on hand-over counts, each of which indicates the number of times of hand-over operations of a mobile terminal from a first connecting base station to one of the plurality of base stations, selecting a second connecting base station from among the relevant base stations based on electric field intensities from the relevant base stations and establishing a connecting link between the mobile station and the second connecting base station.